This invention relates to improvements in blind fasteners and to the joints produced therewith.
Advanced composite materials have recently been gaining increasing usage in various aerospace applications because they offer potential weight savings and increased stiffness. The anisotropic properties of composite material present a unique opportunity to optimize certain structural designs such as elongated aircraft stringers and ribs. In order to form acceptable joints of relatively thin sheet composite materials, conventional blind fasteners have not proven to be completely acceptable. One reason is the low allowable bearing stress of composite materials which means that the full tensile strength of the blind fastener may not be realized because of bearing failure of the composite material. Present blind fasteners have a relatively limited blind side bearing area and typically expand to approximately 1.3 times the original sleeve diameter. Another reason is that composite materials are weaker in shear, thus causing any holes to require greater edge distances and more area buildup to develop full efficiency of the composite material. These materials are further constructed of a resin matrix which has a tendency to crack when holes in the material are expanded, such as by cold working or by an interference fit fastener. Rivets which swell when upset may also crack the resin matrix. Finally, fasteners which form against the blind side composite material surface tend to damage the relatively soft material surface by digging or gouging out some of the material during the bulb forming process.
In order to solve these problems, the present invention is directed to an improved blind fastener assembly which has a large bearing area, does not expand the holes in the respective workpieces in which the fastener assembly is installed, forms away from the blind side workpiece surface, and in which the formation of the blindside bulb is not sensitive to variations in grip length.